The strengthening mechanism for Ni-based superalloy is roughly classified into three kinds, that is, solid solution strengthening, carbide precipitation strengthening, and γ′ (gamma prime)·γ″ (gamma double prime) precipitation strengthening, and among these, γ′-strengthened superalloy utilizing strengthening by γ′ precipitation of an intermetallic compound composed of Ni3Al, Ni3(Al,Ti) or Ni3(Al,Ti,Nb) has been widely used.
The γ′-strengthened Ni-based superalloy exhibits excellent strength properties in a high-temperature environment by virtue of the precipitation of γ′ (gamma prime) working out to a strengthening phase by an aging treatment.
In the case of a γ′-strengthened Ni-based superalloy, the strength at a high temperature can be more enhanced by increasing the γ′ amount. The γ′ amount varies according to the amount added of the forming element such as Al, Ti and Nb, and the precipitation amount can be made large by increasing the amount added of the forming element.
On the other hand, when the γ′ amount is increased by adding the forming element such as Ti, Al and Nb in a large amount, the solid solution temperature of γ′ rises, and the workability at hot forging is worsened. That is, in a γ′-strengthened Ni-based superalloy, the high-temperature strength and the hot forgeability are in a trade-off relationship.
In particular, in the case where the forming element such as Ti, Al and Nb is added in excess of a given amount, the workability becomes so bad that the hot forging can be no longer performed.
Accordingly, an alloy where the forming element such as Ti, Al or Nb is added in excess of a given amount to precipitate a large amount of γ′ phase allows only casting to produce a target member.
However, a member requiring excellent high-temperature strength, for example, a member requiring high strength properties in a high-temperature environment, such as gas turbine of aircraft or for electricity generation, power-generating steam turbine exposed to high-temperature/high-pressure environment typified by A-USC, high output automobile engine component and heat-resistant spring, is preferably formed by forging capable of achieving build-up of a texture via a wrought process, because sufficiently high strength is not obtained by the casting.
In recent years, a material exhibiting excellent high-temperature strength properties while maintaining hot workability has been developed.
For example, a forging alloy excellent in high temperature strength is disclosed in the following Patent Documents 1 and 2.
The alloys disclosed in these Patent Documents can be worked by hot forging but are a hardly workable material.
In the case of a large-sized member such as disc material used in gas turbine, steam turbine and the like, high deformation needs to be added so as to build up the internal texture via a wrought process, but because of difficulty in employing a forging method involving addition of high deformation, it is difficult to apply the hardly workable material to a large-sized member.
Incidentally, in the following Patent Document 3 that is another related art of the present invention, from the standpoint of enhancing the life of a turbine blade, a forged high-corrosion-resistant and heat-resistant superalloy having a composition composed of, in terms of % by weight, C: 0.015% or less, Si: 1.0% or less, Mn: 0.5% or less, Cr: from 15 to 25%, Co: 20% or less, one or two of Mo and W: 7% or less in terms of Mo+½W, Al: from 0.4 to 3%, Ti: from 0.6 to 4%, one or two of Nb and Ta: 6% or less in terms of Nb+½Ta, Re: from 0.05 to 2%, and Fe: 20% or less, and wherein Al+½Ti+¼Nb+⅛Ta is from 2 to 4.5%, with the balance of Ni, is disclosed as an alloy for improving not only the conventional strength but also the resistance to corrosion.
However, all of the alloys described in this Patent Document 3 and the above-described Patent Documents 1 and 2 differ from the superalloy of the present invention in that the amount added of Al as a basic constituent component of γ′ is smaller than in the present invention.
Patent Document 1: US-A1-2003-0213536
Patent Document 2: US-A1-2012-0183432
Patent Document 3: JP-A-119-268337